Method of forming a solder material



United States Patent Office 3,374,093 METHOD OF FORMING A SOLDERMATERIAL Victor J. King, Mountain View, Calif., assignor to LockheedAircraft Corporation, Burbank, Calif.

No Drawing. Continuation of application Ser. No. 434,095, Feb. 19, 1965.This application Apr. 21, 1966, Ser. No. 544,121

2 Claims. (Cl. 75-135) ABSTRACT OF THE DISCLOSURE Method of makingsolder alloy compositions of thallium-indium-mercury which comprises,producing a solid phase in a liquid phase and then decanting said liquidphase.

This application is a continuation of US. application Ser. No. 434,095,filed Feb. 19, 1965, now abandoned, which application was in turn acontinuation-in-part of US. application Ser. No. 196,481, filed May 21,1962, now abandoned.

The preferred alloys which can be formed from thallium, indium andmercury possess unique characteristics which permit the joining ofmetallic as well as non-metallic materials such as refractories,semiconductors, glass, plastic and fabrics.

One of the most critical problems in the art of manufacturingsemiconductive devices concerns the attachment of terminal leads to theexterior surface of a semiconductor body. Hot soldering of the terminalleads to the semiconductor body frequently results in damage to theoperating characteristics of the semiconductive unit. Other approachesto joining, such as spot welding, or pressure welding, likewise inducephysical damage. Any technique which produces excessive heat risks acomposition or phase change in the semiconductive material which willdestroy its utility for its intended application.

In general, the instant invention may be practiced to permit the joiningof terminal leads to semiconductive bodies without altering the physicaland chemical structure of said semiconductive bodies to the point thatthe operating characteristics of the device are seriously impaired. Theinstant invention contemplates the use of an alloy which will effectsound electrical connections between the terminal leads and the surfaceof the semiconductor electrodes, namely, the base electrodes, emitterelectrodes and collector electrodes. This is accomplished by applying anabsolute minimum of pressure and heat to the semiconductivebody duringthe process of joining. For example, when only temporary contacts areneeded for testing and evaluating a semiconductor device, a physicallystrong electrical joint is unnecessary and, indeed, wholly undesirablesince in many instances unsoldering is required at the completion of thetest; consequently, liquid solder is what is needed in such instances.In cases where a semiconductor device will be exposed to very littlephysical strain, the use of high-temperature solders to form exceedinglystrong joints is unnecessary and simply exposes the device to theunnecessary risk of overheating; therefore, utilization of a lowtemperature solder which forms a joint of adequate strength is morerealistic in such circumstances. However, in the event strong joints arerequired it is imperative that when a high temperature solder isusedthat the quantity of heat transmitted to the semiconductor duringthe joining process be kept to an absolute minimum; consequently, apre-tinning alloy which will permit joining of terminal leads to thesemiconductive body by merely applying a drop of high-temperature solderwithout actually applying a soldering iron 3,374,693 Patented Mar. 19,1968 to the semiconductor surface to sweat in the leads is highlydesirable.

The alloys of the present invention are capable of being adjusted incomposition'to perform all of the above-mentioned functions and,moreover, provide joints of exceedingly low contact resistance. To wit,the preferred alloys of the present invention can be made to form: aliquid, low-resistance alloy for temporarily connecting terminal leadsto semiconductive bodies; a solid alloy for low-temperature soldering toform permanent joints; and a liquid pre-tinning alloy of unusual wettingtendencies whereby soldering with high-strength, high-temperaturesolders is facilitated because unwarranted application of heat to thesemiconductor body is avoided. The preparation of a liquid or solidalloy of the present invention for the above-mentioned specificapplications will be hereinafter set forth in detail.

An object of the present invention is, therefore, to provide an alloywhich can be applied to metallic as well as non-metallic materials.

A further object of the invention is to produce alloys which can beapplied to metals as well as non-metals and will exhibit excellentadhesion characteristics.

Still another object is to produce an alloy which at room temperatureforms a liquid of low electrical resistivity suitable for temporaryjoining of metallic as well as non-metallic materials.

Yet another object is to provide an alloy which at room temperatureforms a solid of low electrical resistivity suitable for permanentlyjoining metallic as well as non-metallic materials.

Another object is to provide an alloy which at room temperature willform a liquid exhibiting unusual wetting tendencies on metallic andnon-metallic surfaces.

Another object is to provide an alloy of low ohmic contact resistancesuitable for joining terminal leads to the surface of semiconductiveelectrodes and other electronic components made of metallic ornon-metallic materials.

These objects of the invention are accomplished by providing an alloycomprised of thallium containing either or both indium and mercury. Itwas discovered in the development of the instant invention that alloysof thallium and indium could be made which would produce joints ofsuitable contact conductivity and strength; and likewise alloys ofmercury and thallium resulted in reasonably good joints insofar aselectrical and physical characteristics were concerned. However, formost applications it is usually preferred to form a ternary alloy ofthallium, indium and mercury. In this way, it was found that the mostremarkable and unexpected results of the instant invention resulted. Itwas discovered that an alloy could be formed in which the resultantliquid phase, when applied to an N or P type lead telluridesemiconductor, exhibited unusual wetting tendencies as well as a contactresistance in the order of 3X10" ohms/cm. or less. The alloy in theliquid form can be used, for example, for temporary contacts that can beachieved by touch, by pressure, or rubbing of the semiconductor surfacewith an alloy of preferred composition. It has been found that surfacepreparation in most cases is unnecessary as light oxide surface coatingon the semiconductor did not inhibit the wetting action of the preferredalloys. Although the exact reason for this is not known, it is believedthat this is the result of the natural aflinity which thallium has foroxygen. Consequently, by applicaindium and mercury in the alloy isgreater than the interatomic attraction between these independent atomsin the alloy itself.

The alloy is then in intimate Contact with the semiconduct-or surfacewhose surface atoms may now exert maximum attraction thereby offbalancing the surface tension of the thall-ium-indium-mercury alloy. Theclassical explanation of surface tension postulates that within the bodyof a liquid an atom is acted upon by atoms which are distributed more orless symmetrically about the atom except at the surface. There the atomis only partially surrounded by other atoms, and, as a consequence, itexperiences only an attraction towards the body of the liquid. Thislatter attraction tends to draw the surface atoms inward, and in sodoing makes the liquid behave as if it were surrounded by an invisiblemembrane. However, when the liquid body is placed in contact with adissimilar substance, an interface is formed where the inequalities ofatomic attraction tend to change the shape of the interface.

When the liquid alloys of the present invention are placed on thesurface of a dissimilar substance such as a metallic or non-metallicbody, the cohesive atomic attraction is substantially greater than theadhesive atomic attraction between the atoms in the alloy. Morespecifically, when the instant alloy is present in liquid form on asemiconductive surface, the interatomic attraction between the surfaceatoms of the semiconductor and the atoms in the alloy exceeds theinter-atomic attraction between the atoms of the indium, thallium, andmercury in the alloy itself. Consequently, ubstantially completewettability is effected in much the same way that oil completely wetsthe surface of clean metals.

Likewise, just as the liquid alloys exhibit unusual wettingcharacteristics, the solid alloys of the present invention, when heatedto above their respective melting points, evidence unique capabilitiesfor wetting metals and nonmetallics.

Although the foregoing hypothetical discussion is believed to explainthe phenonemon of unique wettability in the alloys of the presentinvention, it is in no way intended that the invention herein describedis to be so limited.

To produce a typical preferred liquid composition of the instantinvention, a mixture of 33 atomic percent mercury, 20 atomic percentthallium and 47 atomic percent indium is heated to 300 C. to produce asingle-phase liquid. The alloy is then cooled to room temperature,whereupon a solid phase will separate out of the solution. The remainingliquid phase in equilibrium with the solid phase at room temperature canthen be decanted and used for making temporary or easily broken liquidcontacts. Also, the liquid solder can be employed for the purpose ofpretinning surfaces in preparation for hightemperature soldering.

The residue solid phase thus produced can, of course, be used as a solidsolder. However, since at room temperature it would be on the verge ofmelting, it would have low physical strength. It is preferred,therefore, to decant the liquid at a temperature somewhat above roomtemperature to produce an alloy suitable for producing permanent joints.For example, if a terminal lead is to be joined to the electrodes of asemiconductor which cannot be safely heated for prolonged periods toabove 140' C., the liquid-solid slurry should be decanted at atemperature somewhat above 140 C., for example, 200 C. In this way asolid solder will be produced which will not undergo incipient fusioneven though the semiconductor approaches the 140 C. limit.

The above method of producing alloys of the instant invention willnecessarily result in solders which will approximate the compositions ofthe liquidus surface as depicted by the isotherms of a Ti-In-Hg ternaryisomorphous phase diagram. However, the preferred range of compositionof the alloys of the present invention are not to be construed to belimited in such a manner. The desirable properties of wettability andhigh electrical conductivity can be achieved in many other possiblecombinations of the three components, thallium, indium and mercury.

As pointed out hereinabove, suitable binary alloys comprised of eitherthallium and mercury or thallium and indium can be produced which willexhibit suitable properties for many applications. However, when allthree elements are added together to form a ternary alloy it is thenthat the unique desirable wetting characteristics are maximized.

In preparing a binary all-0y from the thallium and indium alone toproduce a preferred embodiment of the present invention, thallium mustbe present from between the limits of 0.1 to 60 atomic percent andindium must be present in the range of from 40 to 99.9 atomic percent.To produce preferred embodiments of the thallium-mercury binary alloy itis necessary that thallium be present in the range from 0.1 to 55 atomicpercent and mercury must be present in the range from 45 to 99.9 atomicpercent. In the first binary com prised of thallium and indium, theresultant solder will be a solid. In the latter, however, when themercury is a predominant component, the thallium and mercury form aliquid alloy suitable for temp-orary contacts or pre-tinning.

Although it would seem that the binary of iridium and mercury would alsoform suitable alloys for purposes of the present invention, it was foundthat a pure binary of indium and mercury without at least 0.1 ofthallium would not achieve the desired results. From this it is seenthat thallium is an essential ingredient in any of the preferred alloysof the present invention whether it be of the binary or ternary class.To produce a ternary utilizing a minimum of thallium, it is necessary toadd at least 0.1 atomic percent thallium to the binary of indium andmercury which, preferably, consist of from 10 to 99.9 atomic percentmercury and of O to atomic percent indium. The maximum thallium, aspointed out above, would be less than 60 atomic percent. Pure thalliumalone does not possess the wetting characteristics desired for theinstant invention.

From the above description of the limits of the preferred alloys in thepresent invention, one is now capable of making a virtually unlimitednumber of specific alloys which are either a binary of thallium andindium or thallium and mercury or a ternary of thallium, indium andmercury, for example, at least 0.1 atomic percent thallium, at least 10atomic percent mercury with the balance being indium, and at least 0.1atomic percent thallium, from 40 to 99.9 atomic percent indium and from45 to 99.9 atomic percent mercury. Whether or not the resultant alloywill be of the liquid or solid variety depends on the proportion ofmercury employed. By using large quantities of mercury it is possible toproduce a ternary alloy possessing exceptional wetting characteristicsand low electrical resistivity which will remain in the liquid conditionto temperatures as low as -35 C. When the solid solder is being made itis desirable to avoid the formation of intermetallic compounds in thealloy, which, by their nature, tend to impart undesirable physicalcharacteristics to the metal matrix.

As pointed out hereinabove, preferred alloys of the present inventionmust containe thallium of at least 0.1 atomic percent. Increasing thethallium content to any point within the ranges set forth hereinabovewill, in general, increase the wettability of the resultant alloy. Nosharp point of demarcation has been observed in increasing the thalliumfrom the very low quantities. Instead a very gradual improvement inwetting capability is observed, up to a point, by gradually increasingthe thallium content of the preferred alloys of this invention. Ingeneral, however, it is preferable that a ternary composition beproduced which will contain from 5 to 25 atomic percent thallium withthe balance being mercury and indium. A typical preferred composition ofthe instant invention consists of 20 percent thallium, 50 percent indiumand 30 percent mercury. This composition produces a liquid alloy capableof readily wetting metals, glasses, plastics, refractories orsemiconductors. For example, a piece of alumina which is lightly brushedwith a few drops of the alloy of this composition is thoroughly andinstantly wetted where contacted. Then, in order to solder a wire to thepiece of alumina, it is preferred to preheat the alumina to 150 C., laythe piece of pretinned copper wire in contact with the wetted surface,drop a globule of molten commercial solder on the area where the wirecontacts the pre-tinned alumina, and allow the joined parts to cool downto room temperature. The joint between a 0.020 diameter wire and a plateof alumina which has been formed by this process can withstand a stressof up to 20,000 pounds per square inch. Another preferred composition ofthe invention consists of 35.6% mercury, 56.4% indium and 8% thallium.

Due to the unique wettability of the alloys herein described, a needlecan be dipped into an alloy of the liquid variety and fine lines drawntherewith on both metallic and non-metallic surfaces. For example, bydrawing a line on a nonconductive substrate with the liquid compositionof the present invention, an electrical circuit can be laid down.Placing the thus inscribed substrate in a heated vacuum environment willvolatilize apart the mercury and thallium thereby leaving intact ametallic electrical circuit.

By providing a non-conductive substrate with connector pins and drawinglines across said substrate between the pins with the liquid alloys ofthe present invention, it is possible for a patchboard programmer in thecomputer art to perform all necessary patchwork at his desk. That is,immediately after calculating which connector pins must be joined forproper patching, he can draw a line with liquid alloys of the presentinvention between the appropriate pins. Then, by hardening the alloysthrough a vacuum treatment, the board can be plugged into the computerto cause the necessary interconnection of electronic units in thecomputer console. In the present state of the art many man-hours arerequired to connect the patchcords to a patchboard for properprogramming; and with the result of many costly errors due to the manualdifficulty of the task.

Another application of the liquid alloys of the present invention is inthe production of low cost printed circuits. One can print a circuitwith the liquid directly on a non-conductive substrate to form acomplete printed circuit upon vacuum treatment. Circuits so produced arethereby manufactured by a simple two-step process which is substantiallysimpler than the present photographic technique used in the printedcircuit art.

It has been found that it is possible to add small quantities of otherelements to the preferred alloys hereinabove described in order toslightly alter the melting point, the electrical conductivity, orwetting characteristics of a specific formulation. Two compatibleadditives which have proven successful in this regard are galium andbismuth. Either can be added to the basic alloy in relatively smallquantities to adjust the above mentioned properties.

It should be understood, therefore, that the foregoing disclosurerelates only to the preferred embodiments of the invention and thatnumerous modifications or alterations may be made therein withoutdeparting from the spirit and scope of the invention as set forth in theappended claims.

Having thus described my invention, What I claim is:

1. The method of forming a solder material comprising the steps of:

(a) forming a mixture of approximately 33 atomic percent mercury,approximately 20 atomic percent thallium, and approximately 47 atomicpercent indium;

(b) heating said mixture to approximately 300 C. to

produce a single phase liquid;

(c) cooling said liquid to room temperature to produce a solid phase insaid liquid;

(d) permitting said solid phase to settle out of said liquid;

(e) and decanting said liquid from said solid phase.

2. The method of forming a solder material comprising the steps of:

(a) forming a mixture of approximately 33 atomic percent mercury,approximately 20 atomic percent thallium, and approximately 47 atomicpercent indium;

(b) heating said mixture to approximately 300 C.;

(c) cooling said mixture to between approximately 140 C. andapproximately 200 C. to produce a solid phase in said liquid;

((1) and decanting said liquid from said solid phase at a temperaturebetween approximately 140 C. and approximately 200 C.

References Cited UNITED STATES PATENTS 3,087,811 4/1963 Freeberg -169FOREIGN PATENTS 1,242,981 8/1960 France.

HYLAND BIZOT, Primary Examiner. RICHARD O. DEAN, Examiner.

